Secondary ion mass spectrometry was used to profile the diffusion of oxygen in polycrystalline β‐cristobalite and vitreous SiO2. The tracer concentration profiles of cristobalite are consistent with a model based on two mechanisms: bulk and short‐circuit diffusion. The profiles of partially crystallized samples containing vitreous SiO2 and β‐cristobalite were fitted using the sum of two complementary error functions and taking account of some interstitial‐network exchange. The bulk oxygen diffusivity, in the temperature range 1240–1500 °C, is about five times greater for vitreous silica than for β‐cristobalite.